The present invention relates generally to methods and apparatus for tubing conveyed perforating, and more specifically relates to methods and apparatus for perforating subsurface formations in a pumped well in response to a pressure differential between the tubing string and the wellbore annulus.
After an oil or gas well has been drilled, casing is typically placed in the well to line the side of the wellbore. Before a formation can be produced, it is necessary to perforate this casing and the formation. Under conventional practices, tubing conveyed perforating guns are lowered into the wellbore until they are in the area of the bore adjacent to the formation. A firing head associated with the perforating guns is then actuated, detonating the perforating guns and perforating the casing and the formation. The perforations allow the gas or oil in the formations to flow into the wellbore annulus. Often it is desirable to have a pressure underbalance between the formation and the wellbore annulus so that when the perforation occurs, the gas or oil in the formation immediately flows into the annulus, flushing out the perforations.
If the drive pressure of the formation is not sufficient to force the gas or oil to the surface through the tubing string, it is necessary to lower a pump into the wellbore to pump the fluids out. The withdrawal of the perforating equipment and placement of the pump requires that the well be killed. In some sensitive formations, once the well has been killed, the formations may not recover to their full, original producing capacity.
Accordingly, the present invention provides a method and apparatus whereby a perforating gun can be actuated by means of a pressure differential between the tubing string and the wellbore annulus with a pump in place in the tubing string, and whereby the formations surrounding the well can be perforated with a desired pressure differential to the well bore. Additionally, the pump may be utilized to establish the pressure differential to actuate the perforating gun.